Investigations are proposed to evaluate the effects of signal level, spread of masking, and aging on speech recognition by normal-hearing and hearing-impaired listeners. The studies proposed will extend our recent work on the application of audibility theory (an expansion of classical articulation theory) to issues underlying hearing aid characteristic selection. Most hearing impaired persons must listen to speech at higher than normal signal levels in order to improve speech recognition. However, the effects of high signal levels on speech recognition performance are not well understood, for either norma1 hearers or hearing- impaired listeners. A clearer understanding of these effects would have direct applications to hearing aid design and characteristic selection. Audibility theory makes unique research designs, data manipulation, and interpretations possible. The proposed investigations are as follows: (1) To develop frequency importance and performance-audibility index transfer functions for a speech test suitable for use in subsequent studies. The test that will be used is the Auditec of St. Louis version of the NU-6 word test. (2) To determine the shape and slope of the performance-intensity function for the NU-6 test in different frequency regions under conditions of constant audibility. Normal-hearing and hearing-impaired listeners will be tested. (3) To investigate the effects of high and low frequency spread of noise masking and speech self-masking on speech recognition in normal-hearing and hearing-impaired ears. (4) To compare performance-intensity functions at masked and unmasked thresholds in normal ears and to evaluate the effects of threshold configuration on the performance-intensity function in hearing-impaired ears. (5) To measure the speech recognition "proficiency' of normal-hearing subjects under conditions of constant audibility as a function of age. (6) To evaluate the audibility index obtained under conditions of constant loudness as a basis for selecting hearing aid frequency response. The results of these various projects will also be used to evaluate the effects of procedural and stimulus variables on measured frequency-importance and transfer functions.